<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>

        //es5
        function fn(a,b){
            var a=a||1;
            var b=b||2;
            console.log(a,b);
        }

        fn(2);


        //es6
        function fn(a=2,b=3){
            console.log(a,b);
        }

        fn(5);

        //短路
        function fn(cb){
            cb && cb();
            //当传入为函数时为真执行后面调用
            //不传入参数为假不执行后面调用
        }
        fn(function(){
            console.log(123);
        })
    </script>
</body>
</html>